Ultra-high vacuum metallic seal

ABSTRACT

A metallic seal design that reduces manufacturing costs by minimizing production steps and also lowers the sealing force by incorporating a thin-wall, high height/width ratio annular column. The seal has two horizontal ribs which constrain the thin-wall column from unstable buckling to thereby reduce or eliminate the tendency of the sealing dams to become inclined to sealing flange surfaces.

RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional application Ser.No. 12/354,578 filed Jan. 15, 2009 which claims the benefit of U.S.Provisional Application No. 61/021,067 filed Jan. 15, 2008, which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a seal for creating a sealbetween a pair of sealing surfaces and, more specifically, to a metallicseal.

BACKGROUND OF THE INVENTION

Elastomeric seals generally are not suitable for ultra high vacuum (UHV)sealing applications because of the inherent open structure of polymericchains through which molecular gaseous species can diffuse. The lowestachievable He leak rate through elastomeric seals is typically 10⁻⁸cc/sec. Using metallic seals, He leak rates in the range of 10⁻⁹-10⁻¹¹cc/sec can be easily obtained. Metallic seals, however, generallyrequire high sealing force (F_(s)), also referred to as mechanicalcontact pressure P_(mc) where P_(mc)=F_(s)/A_(s) and A_(s) is the areaof the sealing dam. If P_(mc) exceeds the yield strength of the sealingflange material, flange surfaces can be brinelled when they compress themetallic seal between them to achieve UHV. If brinelled, the flangesurfaces will typically require reconditioning before installing newseals. Therefore, metallic seals have been designed to reduce the forcerequired to compress the seal by optimizing seal cross section.

Metallic seals can also offer a longer seal life compared to elastomericseals in applications where process chemicals would otherwise degrade anelastomeric material, for example in semiconductor processingapplications. Elastomeric seals are attacked by highly reactive radicalssuch as NF₃ and O₂ which severely damage the polymeric chain structurethereby limiting the seal life. Metallic seals made from nickel,aluminum, tin, and/or stainless steel, for example, can be used inappropriate environments in which specific alloys are found inert.Because of extremely low leakage characteristics, metallic seals areoften used to seal poisonous gases, such as PH₃, commonly used insemiconductor processing. Metallic seals of appropriate design canachieve leak rates even lower than welded joints. For example, He canhave a higher molecular diffusion rate through weld defects than througha metallic seal due to micro cracks, grain boundaries and/or porosity ofthe welds.

U.S. Pat. No. 6,409,180 to Spence et al. (“Spence”) discloses a UHVmetallic seal design similar to the one shown in FIG. 1. The Spence sealconsists of four sections: two beams 1 and 2, a column 3, two diagonalbraces 4 and 5, and two sealing dams 6 and 7. The diagonal brace angleis 35 to 55 degrees. An recessed surface ABC between beams 1 and 2 formsvariable width column 3 having a minimum width at the center.

When the flanges 9 and 10 compress the seal, the seal height is reducedas the column 3 undergoes stable buckling maintaining the sealing damsurfaces 6 and 7 parallel to the sealing surfaces of flange 9 and 10, asillustrated in FIG. 2.

A number of steps are typically necessary for machining the Spence sealsfrom a hollow tube. For example, and with reference to FIG. 3, onemethod of making the Spence seal may include:

-   -   (i) machining an annular recessed surface ABC with a tool T1        having the desired profile;    -   (ii) sectioning the individual seals,    -   (iii) machining the first sealing dam with a second tool T2; and    -   (iv) machining the second sealing dam.

An alternative method might include:

-   -   (i) machining the annular recessed surface ABC with a first tool        T1;    -   (ii) machining another annular recess with a second tool T2 to        form the sealing dams; and    -   (iii) sectioning individual seals.

There are several disadvantages of the Spence seal design. For example,the large number of machining steps can increase production cost, andthe variable thickness column including the braced section requires ahigh sealing force (F_(s)) and P_(mc) which can cause brinelling of thesealing surfaces of the flanges.

SUMMARY OF THE INVENTION

The present invention includes a seal design that reduces manufacturingcosts by minimizing production steps and also lowers the sealing forceby incorporating a thin-wall, high height/width ratio annular column.The seal has two horizontal ribs which constrain the thin-wall columnfrom unstable buckling to thereby reduce or eliminate the tendency ofthe sealing dams to become inclined to sealing flange surfaces.

Accordingly, a metallic seal for sealing axially facing surfacescomprises an annular column portion surrounding a central axis andhaving at opposite axial ends respective sealing dams for engaging theaxially facing surfaces to be sealed. At least two ribs extend radiallyfrom the annular column portion, the ribs being axially spaced apartfrom each other and each pair of ribs defining therebetween an annularrecess. The ribs are spaced at a pitch having a length generallycorresponding to an axial length of the metallic seal divided by thenumber of ribs. For example, for a seal having a length L and two ribsthe pitch of the ribs would be L/2.

More particularly, the recess can be generally U-shape, and the ribs canextend perpendicular to the central axis. The annular column portion canextend axially beyond respective axially outermost ribs thereby formingrecesses that correspond in shape to one half of the annular recess. Theannular column portion can have radially inner and outer surfacesdefining a column width therebetween, the column width beingsubstantially uniform along the height of the annular column portion,and the column portion may have a height/width ration of about seven orgreater. At least one rib can have a thickness dimension in the axialdirection that is about equal to or greater than a radial width of thecolumn. The respective sealing dams can be in axial alignment with theannular column portion and may have a radial width between about 0.008and 0.016 inches.

In accordance with another aspect, a method of making a metallic sealhaving an annular column portion surrounding a central longitudinal axiswith respective sealing dams on ends thereof for sealing axially facingsurfaces, and at least two ribs extending radially from the annularcolumn portion is provided. The method includes forming a tubular sealblank having at least three equally axially spaced apart circumferentialrecesses of essentially the same shape forming therebetween respectiveradially extending ribs, and severing the tubular seal blank along theaxial midpoint of the recesses that have disposed therebetween at leasttwo of the radially extending ribs, to thereby separate at least onemetallic seal from the seal blank. The recesses can be U-shape and theribs can extend perpendicular to the central axis. The recesses can beformed by a machining process, for example.

In accordance with another aspect, a metallic seal for sealing axiallyfacing surfaces comprises an annular column portion surrounding acentral axis and having at opposite axial ends respective sealing damsfor engaging the axially facing surfaces to be sealed, and at least tworibs extending radially from the column portion, the ribs being axiallyspaced apart from each other and defining an annular recess between eachpair of relatively adjacent ribs, and the axial ends of the annularcolumn portion extending axially beyond the respective axially outermostribs and defining therewith respective recesses relative to therespective sealing dams, and each recess corresponds in shape to onehalf of the annular recess.

The recess can be generally U-shape, and the ribs can extendperpendicular to the central axis. The annular column portion can haveradially inner and outer surfaces defining a column width therebetween,the column width being substantially uniform along the height of theannular column portion. At least one rib can have a thickness dimensionin the axial direction that is about equal to or greater than a radialcolumn width.

Further features of the invention will become apparent from thefollowing detailed description when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of prior art metallic seal in anuncompressed state.

FIG. 2 is a cross-sectional view of the prior art seal of FIG. 1 in acompressed state.

FIG. 3 is a schematic illustration of a prior art method of making theseal of FIGS. 1 and 2.

FIG. 4 is a cutaway three-dimensional view of an exemplary metallic sealin accordance with the invention

FIG. 5 is a cross-sectional view of the metallic seal of FIG. 4 in anuncompressed state.

FIG. 6 is a cross-sectional view of the metallic seal of FIG. 4 in acompressed state.

FIG. 7 is a schematic illustration of an exemplary method of making theseal of FIGS. 4-6 in accordance with the invention.

FIG. 8 is a perspective view of a seal assembly including a retainerplate and a pair of exemplary metallic seals in accordance with theinvention.

DETAILED DESCRIPTION

Turning now to FIGS. 4-7, and initially to FIG. 4, an exemplary metallicseal in accordance with the invention is generally indicated byreference numeral 20. The ring shape seal includes an annular columnportion 24 surrounding a central axis A and having at opposite axialends respective sealing dams 28 and 32 for engaging axially facingsurfaces to be sealed, which as shown in FIGS. 5 and 6 are sealingflanges 36 and 40. Two ribs 44 extend radially from the column portion24.

Turning to FIG. 5, the ribs 44 are axially spaced apart from each otherand define therebetween an annular recess 52. The recess 52 in theillustrated embodiment is generally U-shape, and the ribs 44 generallyextend perpendicular to the central axis A. The column portion extendsaxially beyond respective ribs 44 and define therewith respectiverecesses 53 that correspond in shape to one half of the annular recess52.

As will be appreciated, the ribs 44 are axially spaced apart at a pitchP having a length generally corresponding to an uncompressed axiallength L of the metallic seal 20 divided by the number of ribs of theseal. Thus, in the illustrated embodiment, the pitch P is generallyequal to about the axial length L divided by two. In a seal having threeribs, the pitch P would be equal to about the axial length L divided bythree, and so on for seals of having additional ribs. Thus, the axialextent of a base 54 of the recess 52 in the illustrated embodiment isapproximately twice the axial extent of the terminal portions of thecolumn 24 extending axially beyond respective ribs 44 and/or the axialextent of recesses 53.

A radially inner surface 58 and a radially outer surface 60 of theannular column portion 24 define a column width Cw therebetween, withthe column width Cw being substantially uniform along the axial length Lof the annular column portion 24. Each metallic rib 44 in theillustrated embodiment has a thickness dimension t in the axialdirection that is about equal to or greater than the width Cw of theannular column portion 24. As will be appreciated, other dimensions ofthe annular column portion 24, recess 52, and ribs 44, and/orrelationships therebetween, are possible.

The sealing dams 28 and 32 are each in axial alignment with the annularcolumn portion 24, with the column width Cw being the same as thesealing dam width. Thus, the annular column portion 24 virtually extendsfrom one sealing surface to the other sealing surface. This results in aseal 20 having sealing dams 28 and 32 with the same width as constantwidth column 24 and being in axially alignment therewith.

As will be appreciated, the height/width ratio of the column 24 in theillustrated embodiment is generally the axial length L (height) of theseal divided by the column width Cw (width). Seals in accordance withthe invention can have a column height/width ratio of wide range, forexample seven (7) or greater. By increasing the height/width ratio ofthe thin-wall column 24 the sealing force (F_(s)) is reduced for thesame amount of compression of the seal free height.

In FIG. 6, the seal 20 is shown in a compressed state with sealing dams28 and 32 parallel to surfaces 36 and 40 thereby providing a tight seal.The annular column portion 24 is buckled in a controlled mannergenerally around a midpoint of the axial extent of the recess 52. Thecontrolled buckling maintains the sealing dams 28 and 32 parallel to thesealing surfaces and/or each other to facilitate a tight seal and toreduce or eliminate relative movement between the sealing dams 28 and 32and surfaces 36 and 40.

Turning to FIG. 7, a method for making the seal 20 will be described. Ingeneral the method begins with forming a tubular seal blank 70 having atleast three equally axially spaced apart circumferential recesses 52 ofessentially the same shape. The recesses 52 form therebetween respectiveradially extending ribs 44. As will be appreciated the recesses 52 canbe formed by any suitable process, such as by machining, for example. Asillustrated, a tool T3 is shown for machining the recesses 52.

Once the tubular blank 70 and/or recesses 52 are formed, the tubularblank 70 is then severed along the axial midpoint M of the recesses 70that have disposed therebetween at least two of the radially extendingribs 78, thereby separating a metallic seal 20 from the seal blank 70.

Accordingly, the seal 20 can be made in as few as two process steps.First the annular recesses 52 are machined using a single tool, followedby sectioning in the middle of alternate grooves to produce individualseals as illustrated. The resulting seal 20 has an annular outer surfaceABC that consists of three sections: one perfectly vertical definingabout 50% of the column height, and two perfectly horizontal surfacesextending radially outward defining a part of each rib. Unlike prior artseals that may have a variable column width, seal 20 has a column ofuniform width without any diagonal brace section. The annular columnwidth Cw is also the same as the sealing dam width which is in alignmentwith the annular column 24 such that the column structure (e.g., annularcolumn 24 and sealing dams 28 and 32) virtually extends from one sealingsurface to the other sealing surface. This increases the height/widthratio of the thin-wall column and reduces the sealing force (F_(s)) forthe same amount of compression of the seal free height.

Turning now to FIG. 8, a seal assembly includes a seal retainer 104 andtwo metallic seals 20. As will be appreciated, the seal retainer 104supports and locates the metallic seals 20 for ease of assembly.Accordingly, the seal retainer 104 can generally be formed from sheetmetal or the like and includes holes 108 through which bolts or otherthreaded fasteners can pass. The retainer 104 also has cutout portions112 into which the metallic seal 20 are inserted before being slid intothe position shown. Retaining arms 116 are provided for securing themetallic seal 20 in the position shown in FIG. 8. As will beappreciated, the retainer 104 not only supports and locates the metallicseals 20 for ease of assembly, but may also serve as a spacer to preventovercompression of the metallic seals 20 during assembly.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described elements (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such elements are intended to correspond,unless otherwise indicated, to any element which performs the specifiedfunction of the described element (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one or more of several illustrated embodiments, suchfeature may be combined with one or more other features of the otherembodiments, as may be desired and advantageous for any given orparticular application.

1-9. (canceled)
 10. A method of making a metallic seal having an annular column portion surrounding a central longitudinal axis with respective sealing dams on ends thereof for sealing axially facing surfaces, and at least two ribs extending radially from the annular column portion, the method comprising: forming a tubular seal blank having at least three equally axially spaced apart circumferential recesses of essentially the same shape forming therebetween respective radially extending ribs; and severing the tubular seal blank along the axial midpoint of the recesses that have disposed therebetween at least two of the radially extending ribs, to thereby separate at least one metallic seal from the seal blank.
 11. A method as set forth in claim 10, wherein the recesses are U-shape, and the ribs extend perpendicular to the central axis.
 12. A method as set forth in claim 10, wherein the recesses are formed by a machining process.
 13. A method as set forth in claim 10, wherein the annular column portion extends axially beyond respective axially outermost ribs forming recesses that correspond in shape to one half of the annular recess.
 14. A method as set forth in claim 10, wherein the annular column portion has a height dimension along the central axis, and wherein the annular column portion has a radially inner surface and a radially outer surface, the column inner and outer surfaces defining a column width therebetween, the column width being substantially uniform along the height of the annular column portion.
 15. A method as set forth in claim 10, wherein the annular column portion has a radially inner surface and a radially outer surface, the column inner and outer surfaces defining a column width therebetween, and wherein the first and second rib portions have a thickness dimension in the axial direction that is about equal to or greater than the column width.
 16. A method as set forth in claim 10, wherein the respective sealing dams are axially aligned with the annular column portion.
 17. A seal as set forth in claim 10, wherein at least one of the sealing dams have a radial width between about 0.008 and 0.016 inches. 18-20. (canceled) 